Combined microphonic transmitter and amplifier therefor



I May 27, 1930. THOMPSON 1,760,672

COMBINED MICROPHONIC TRANSMITTER AND AMPLIFIER THEREFOR Filed July 21, 192'! INVENTOR Lin C0! ThOmpsn I ATTORNEY Patented May 27 19 UNITED STATES PATENT OFFECE I LINCOLN THOMPSON, OF NAUG-ATUCK, CONNECTICUT, ASSIGNOR, BY MESNE ASSIGN- MENTS, TO WM. H. BRISTOL TALKING PICTURE CORPORATION, OF WATERBURY, i CONNECTICUT, A CORPORATION OF CONNECTICUT COMBINED MIICROPHONIC TRANSMITTER AND AMPLIFIER THEREFOR Application filed July 21,

The invention relates to apparatus for reproducing sound, more particularly apparatus of the microphonic type; and it has for its object to associate with a microphone of any well-known or special design amplifying means which will not only materially increase the volume of sound resulting from the actuation of the microphone, but at the same time will furnish conveniently the source of energy necessary in the operation of the microphone' and without appreciable distortion of the resulting sound waves. The invention has for its object, furthermore, to so arrange the supply of energy for the microphone that the operation of the amplifier itself is simplified in that it provides at the same time for a biasing of the grid of said amplifier. A further object of the invention resides in the provision of means for varying the volume of the output; also, in the limiting of the voltage across the terminals of the energizing circuits for the microphone, so that objectionable voltages with consequent shocks to one coming in contact therewith willnot result when the said energizing circuit is open.

To this end, the invention contemplates connecting the energizing circuit of the microphone in series with the plate circuit of the amplifier, suitable resistances being associated therewith as will hereinafter more fully appear.

The nature of the invention, however, will best be understood when described in connection with the accompanying drawings, in which: 7

Fig. 1 is a front elevation'illustrating a microphone device suitable for use with phonographic recordsas one means for actuating the sameand indicated as associated with a one-stage audio frequency amplifier to which it is connected by suitable conductors. 'Fig. 2 is a dlagrammatlc representation of the circuits and connections mvolved.

Referring to the drawings, 10 designates 1927. Serial No. 207,372.

having a stylus 11 adapted to engage grooves of a sound record 12 and as is well understood in the art. This box is ordinarily pivoted to a strap having a suitably supported.

arm 13, and through the latter conductors are led to further apparatus for amplifying and converting the fluctuations in the electric current produced by the microphone into sound waves. The particular microphone herein disclosed, reference being had more especially to Fig. 2 of the drawings, is of the type shown in U. S. Letters Patent No. l .590,7 to A. P. Pinkler, and comprises the microphone elements 15, transformer 16 and output leads 1.7 therefrom.

Ordinarily, a source of potential, such as a 1 or 3-volt battery, is included in the leads 18 of the energizing-circuit for the microphone; but in accordance with the pres ent invention, the supply of energy is to be derived from the plate circuit of an audio frequency amplifier 19. The latter may be, generally, of any well-known or standard type embodying the thermionic valve 20, the A-battery 21 for the filament thereof and the B-battery 22 for the plate circuit of said amplifier, also an input transformer 23 and herein indicated as of the auto-transformer type the order of 1,000 ohms introducedin series substantially at the one terminal, said terminals being adapted to be connectedto the leads 18 of the energizing circuit for the microphone. This may conveniently be effected by the double-throw switch 27, the other pair of terminals 28 of which are designed to connect in a'secondary source of It is desirable, also, that the imt cuit will also be open, and, furthermore,'

there will be set up across said terminals a substantial potential which might cause objectionable shocks should: one operating the apparatus come in contact therewith.

There is, therefore, provided to bridge said terminals a non-inductive resistance of considerably higher value than the resistance 26, for example, of the order of 2,000 ohms; and a connection as the lead'31 is arranged to short-circuit the same when switch 27 is thrown to connect in the battery 29 in order that the grid bias may not become too high. Moreover, during the interval of switching it affords protection in always preventing high voltage leads from extending outside the amplifier. The said resistance will serve, also, to keep the terminal volt-ages on open circuit comparatively low (less than volts), and as the energizing circuit resistance is comparatively low (say less than 100 ohms) no appreciable current will be taken thereby,

from the energizing circuit of the microphone. Furthermore, the changing plate current would ordinarily cause-distortion by the varying voltages applied to the microphone; but this is largely overcome by the comparatively high resistance 26 and, furthermore, there is provided a by-passing condenser 32, for example, of the order of 4 mfd., thus keeping the energizing current for the microphone substantially constant.

It is desirable, also, to'provide means for varying the volume of output; and t-o this end, a variable non-inductive potentiometer 33ranging from 0 to 50,000 ohms is included across the input transformer 23, the same being regulated by a knob 34 at the front of the amplifier 19 which is provided, also, with a knob 35 for the control of the filament current through resistance 36 in circuit with the filament of the thermionic valve 20.

The foregoing combination thus provides for a simple, compact and self-contained, as well as portable, unit 19 associated with the sound box or microphone member 10; and.

by the novel arrangement of circuits shown and described, the sound waves reproduced thereby are comparatively free from distortion. of the microphone transformer and amplifier input transformer, the energy required for operating the former is kept comparatively low, thus making forbetter operation of said Moreover, by matching the impedances microphone. By matching the impedance of the microphone transformer (one side of which matches the microphone used and the other side matching the average loud speaking device), with the input side of the autotransformer of the amplifier, the auto-transformer step-up may be made very high with.- out getting distortion and consequently the low microphonecurrent for the plate circuit allows the microphone out-put to be comparatively small although due to .this high transformer voltage step-up, the amplifier output is very large, v

I claim:

1. In combination: a microphonic transmitter; an audio frequency amplifier connected therewith; and an energizing circuit for the microphone in series with the plate circuit of the amplifier. i

2. In combination: a microphonic transmitter; an audio frequency amplifier connected therewith; and an energizing circuit for the microphone in series with the plate circuit of the amplifier and a substantial series resistance.

3. In combination: a microphonic trans mitter; an audio frequency amplifier connected therewith; and an energizing circuit for the microphone in series with the plate circuit of the amplifier and a resistance of the order of 1000 ohms. Y

4:. In combination: a microphonic trans-- mitter; an audio. frequency amplifier connected therewith; an energizing circuit for the microphone in series with the plate circuit of the amplifier and a substantial series resistance; and a resistance bridged across the terminals of the energizing circuit from.

the amplifier.

5. In combination: a microphonic trans the microphone in series with theplate cir- I cuit of the amplifier and a substantial series resistance; and aby-passing condenser for the said resistance.

7. In combination: a microphonic trans mitter; an audio frequency amplifier connected therewith; an energizing circuit for the microphone 1n series w th the plate circuit of the amplifier and EL'SGI'lBS resistance of the order of 1,000 ohms; and a byassing condenser of the order of 4 mfd. for t e said resistance.

8. In combination: a microphonic transmitter; an audio frequency amplifier connected therewith; an energizing circuit for the microphone in series with the plate circuit of the amplifier; a supplementary source of potential; and a switch to connect at will to the microphone of said secondarysource of potential or the energizing supply from the amplifier without opening said plate circuit.

9. In combination: a microphonic transmitter; an audio frequency amplifier connected therewith; an energizing circuit for the microphone in series with the plate circuit of the amplifier and a substantial series resistance; a bridging resistance; a supplemental-y source of potential; a switch to connect at will to the microphone said secondary source of potential or the energizing supply from the amplifier; and means to short-circuit said bridging resistance when the supplementary source of potential is connected to the microphone.

"In testimony whereof I affix my signature.

LINCOLN THOMPSON. 

